Interpretive Summary: Postharvest fruit rot diseases result in significant economic losses of apples during storage or in the market. Sphaeropsis rot caused by the fungus Sphaeropsis pyriputrescens is a recently reported postharvest disease of apple. The goal of this study was to identify fungicides that can be used as a preharvest spray to control Sphaeropsis rot in stored apple fruit. Three registered fungicides Pristine, Topsin M and Ziram were tested for inhibitory effects on mycelial growth of the fungus in the laboratory. In the orchard tests, ‘Golden Delicious’ apple fruit were inoculated with the conidial suspension of the fungus at 2 or 5 weeks before harvest, sprayed with fungicides within 2 weeks before harvest, and harvested and stored at 0ºC for disease evaluation. All three fungicides effectively inhibited mycelial growth of the fungus in the in vitro tests. On apple fruit in four seasons, Pristine applied 1 week and Ziram applied 2 weeks before harvest significantly reduced incidence of Sphaeropsis rot compared to the nontreated control by 43 to 80% and 42 to 83%, respectively. The performance of Topsin M was less consistent compared with Pristine and Ziram.

Technical Abstract:
Sphaeropsis rot caused by Sphaeropsis pyriputrescens is a recently reported postharvest fruit rot disease of apple in Washington State and causes significant economic losses. Infection of apple fruit by the fungus occurs in the orchard, but decay symptoms develop during storage or in the market. The objective of this study was to evaluate preharvest fungicide application to control Sphaeropsis rot. In vitro sensitivity to registered fungicides, Pristine, Topsin M and Ziram was tested for mycelial growth with 30 isolates of the fungus collected from various sources. In the orchard, ‘Golden Delicious’ apple fruit were inoculated with the conidial suspension of the fungus at 2 or 5 weeks before harvest, sprayed with fungicides within 2 weeks before harvest, and harvested and stored at 0ºC for disease evaluation. All three fungicides effectively inhibited mycelial growth of the fungus in the in vitro tests. On apple fruit in four seasons, Pristine applied 1 week and Ziram applied 2 weeks before harvest significantly reduced incidence of Sphaeropsis rot compared to the nontreated control by 43 to 80% and 42 to 83%, respectively. In the four seasons, the performance of Topsin M was less consistent compared with Pristine and Ziram.